快速幂算法

3^10=3*3*3*3*3*3*3*3*3*3
3^10=(3*3)*(3*3)*(3*3)*(3*3)*(3*3)
3^10=(3*3)^5
3^10=9^5

9^5=（9^4）*（9^1）
9^5=（6561^1）*(9^1)

long long fastPower(long long base, long long power, long long mod) {
    long long result = 1;
    while (power > 0) {
        if (power & 1) {//此处等价于if(power%2==1)
            result = result * base % mod;
        }
        power >>= 1;//此处等价于power=power/2
        base = (base * base) % mod;
    }
    return result;
}